Prediction of a hexagonal SiO2 phase affecting stabilities of MgSiO3 and CaSiO3 at multimegabar pressures.

Abstract

Ultrahigh-pressure phase relationship of SiO(2) silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe(2)P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe(2)P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO(9) tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO(2) on the stability of MgSiO(3) and CaSiO(3) at multimegabar pressures. A postperovskite phase of MgSiO(3) breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe(2)P-type SiO(2) and CsCl (B2)-type MgO. CaSiO(3) perovskite, on the other hand, directly dissociates into SiO(2) and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO(2), MgSiO(3), and CaSiO(3) indicate that the Fe(2)P-type SiO(2) could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.